• In small bowl mix flour, baking powder, and salt together. Set aside.
• In mixing bowl cream butter and sugar until light and fluffy.
• Beat in eggs, one at a time.
• Add vanilla and sour cream, gently beat just until combined.
• Beat in a small amount of the flour mixture followed by a small amount of milk. Repeat until all the flour and milk is added.
• Just beat until mixed.

To create the separate batters

• Divide the batter in half.
• Add cocoa to one half of the batter. Stir until incorporated.
• Add mini chocolate chips to the other half of the batter (plain). Stir until incorporated.

How to Marble:

• In a greased and floured bundt pan drop batter by the spoonful, alternating chocolate and vanilla flavors. Use only 1/3 of the batter for the first layer. (this batter is thick, don't worry about that...it will work out in the oven!)
• Drop spoonfuls again but this time alternate the flavors. So, chocolate is now on top of a spoonful of vanilla, and vanilla is on top of a chocolate. 
• Drag a butter knife through the cake going across the width of the cake, and back again. Do not be tempted to "over swirl" or you'll get a muddy looking cake. Wipe the knife off when you get to the end.
• Now, swirl the cake down the length of the pan, again do not over-do it.
• BAKE the cake in a 350 degree oven for about 50-60 minutes (check at 40 minutes -- overbaking will dry this cake out) or until a knife inserted comes out clean.